There are a number of situations in which it might be desirable to control ingress of water into a container until a large amount of water is present. For example, it may be required to have a liquid-activated trigger mechanism for automatic inflation of a life jacket or other maritime life preservation device, for example, or for activation of a flood warning system. In such circumstances, it may also be preferable to protect against activation of the trigger by mere splashing or contact with rain, even where large quantities of water are involved. Ideally, activation of the trigger might be preferred or desirable only in the event of complete immersion of the system.
In the case of a life jacket, in existing systems a steel CO2 container is typically utilised for release of CO2 to rapidly inflate the life jacket. To ensure activation, a metal spring exerts a force of over 200N on a 2-3 mm diameter pin needed to puncture the steel CO2 container. Due to the extremely small space available, in compression this spring is exerting a force of approximately 300N in the ‘armed’ position. In order to resist this force a solid substance is required to intervene between the spring and surrounding support structure. To allow activation, the solid substance must be able to fail quickly on contact with water. Common substances used are a pellet of compressed dissolvable powder material or a highly compressed paper drum similar to tissue paper. Both, by their very nature, are hydrophilic, so any minor water ingress can result in a fail and subsequent firing. Therefore, servicing at regular intervals is essential if false activation through age is to be avoided. Even with such maintenance, false life jacket inflation is a common occurrence.
Furthermore, due to the need for instant inflation when the wearer enters the water, automatic lifejackets rely on diverting water away from the activator in one direction only, to counteract the effect of rain and spray dripping downwardly in. This means that frequently, if the wearer is sitting and water spray comes up from beneath the wearer, false activation can occur.
In recent years, the introduction of the hydrostatic activation system has prevented many false activations. In these devices, the trigger will not fire until the system is submerged, creating a positive water pressure typically found with a depth of at least 10 cm of water. However, these systems have two main disadvantages. The first is the additional cost of a replacement trigger head, of two and four times the cost of a conventional unit. The second (and more serious) disadvantage is that, if the casualty is wearing buoyant clothing, especially where buoyancy is provided to the lower torso or legs, the activation may be delayed due to an insufficient immersion depth.
The device described in WO2016/020649 is an additional system which may optionally be packed within a lifejacket, which enables easy contact to be made with a person who has fallen overboard from a vessel. A would-be rescuer can make safe initial contact with the victim without jeopardising their own safety, securing the victim to the vessel prior to attempting to bring the victim back on board. One component of this system is a buoyant target element, with which the rescuer first makes contact when executing a rescue manoeuvre. The whole device is packaged within a typical lifejacket, but it may be preferable to deploy the buoyant target element separately from the deployment of the life-jacket itself. Therefore, it is desirable to identify a way of controlling the deployment of this element only when the wearer is immersed in water, rather than accidentally due to contact with waves or rain.